1. Field of the Invention
The present invention relates to an optical fiber built-in type composite insulator having an end portion sealed by an inorganic glass.
2. Related Art Statement
Heretofore, various structures of optical fiber built-in type composite insulator have been known. Among them, the structure of sealing the optical fiber at the both ends of penetration hole of an insulator with an inorganic glass has to bond a cladding of the optical and the inorganic glass to each other, so that the portion of the optical fiber which bonds the inorganic glass has to be removed of its coating before the sealing.
Namely, an example of such structure is shown in the attached FIG. 3 wherein, when inserting an optical fiber 23 in a penetration hole 22 penetrating through the central axis portion of an insulator 2, the optical fiber 23 having a coating portion 23-1 is fixed at the middle portion of the penetration hole 22 by filling a rubbery elastomer or a resin, such as silicone rubber or epoxy resin, and a coating-removed or stripped portion 23-2 of the optical fiber 23 is sealed in the both end portions of the penetration hole 22 by means of an inorganic glass 25 to extend the coating portion 23-1 from the inorganic glass 25. The coating-stripped portion 23-2 and the coating portion 23-1 of the optical fiber 23 extending from the inorganic glass 23 to the exterior are directly protected by a protective layer 26 made of a rubbery elastomer or a resin, such as silicone rubber or epoxy resin. The coating-stripped portion 23-2 and the coating portion 23-1 of the optical fiber 23 extending from the inorganic glass 25 to the interior are protected by an inorganic or heat resistant organic adhesive layer 27 filled between the inorganic glass 25 and the middle portion of the penetration hole 22.
However, the abovementioned structure of the optical fiber built-in type composite insulator has a problem in that the optical fiber 23 is damaged at the coating portion 23-1 and/or the stripped portion 23-2, as described below.
Namely, as the stresses exerted on the optical fiber 23 from the exterior, there are forces unavoidably exerted during the sealing operation or the end-protecting operation, a stress accompanying expansion and shrinkage of the rubbery elastomer or resin 24 in the middle portion and the protective layer 26 of the penetration hole 22, and a force accompanying the handling of the optical fiber 23, such as fusion splicing, etc., after finishing the protection of the end portion of the penetration hole 22.
Therefore, there is a problem in that after the finishing of the sealing by means of the inorganic glass 25 and before the finishing of the protection of the coating-stripped portion 23-2 and the coating portion 23-1 extending from the sealing portion to the exterior, the optical fiber 23 is damaged due to an unavoidable force generated during the operation of forming the protection. There is also another problem in that, in case when the coating-stripped portion 23-2 and the coating portion 23-1 extending from the sealing portion of the inorganic glass 25 to the exterior were directly protected by the protective layer 26 (e.g., rubbery elastomer or resin), the extended portion of the optical fiber 23, particularly the coating-stripped portion 23-2 of weak mechanical strength is damaged by a stress generated by expansion and shrinkage of the protective layer 26 due to temperature change during use.
There is also a further problem in that, in case when the coating-stripped portion 23-2 and the coating portion 23-1 extending from the sealing inorganic glass portion 25 to the interior were protected by the heat resistant organic adhesive layer 27, the optical fiber 23 is similarly damaged. Meanwhile, in case when the extended portion of the optical fiber 23 was protected by the inorganic adhesive layer 27, there is a problem in that the inorganic adhesive layer 27 is fragile and liable to entrain air bubbles therein, so that it can not firmly fix the optical fiber 23 sufficiently, and the optical fiber 23, particularly the coating-stripped portion 23-2 of weak mechanical strength, is exerted by a stress generated by expansion and shrinkage of the rubbery elastomer or resin 24 filled in the middle portion of the penetration hole 22 due to temperature change during use, so that the optical fiber is also damaged.
Furthermore, there is also a problem in that, accompanying the expansion and shrinkage of the rubbery elastomer or resin 24 filled in the middle portion of the penetration hole 22, a bending stress is exerted on the portion of the optical fiber 23 existing on the interface between the adhesive 27 and the rubbery elastomer or resin 24, so that the optical fiber 23 is damaged.